This invention relates to radio frequency interference suppression means which attenuate unwanted radio frequency interference associated with the firing of a spark plug and the like and, particularly, to ignition wiring systems for internal combustion engines associated therewith.
Spark-ignited internal combustion engines generally employ a separate spark plug for each cylinder to ignite the fuel mixture. The spark plug generally employs a pair of electrodes defining a gap across which a spark is initiated by the application of a high voltage.
Multiple cylinder engines of the above type have generally employed a distributor to direct the high voltage output of the ignition coil to the proper one of the spark plugs. A lead from the ignition coil connects the output of the coil to a rotating distributor electrode via a carbon brush riding on the electrode. The rotating electrode is mechanically synchronized to the engine, and makes a close physical approach to, but normally does not touch, the fixed distributor electrodes. Each fixed electrode is connected by a lead to an associated spark plug. When a selected spark plug is to be fired, the high voltage output from the ignition coil sparks across from the rotating electrode to the proper fixed electrode, thereby establishing a path from the ignition coil to the selected spark plug.
Sparks generated in modern ignition systems constitute relatively strong radio frequency interference sources. The strongest interference source is generally the spark plug, because of the high voltage required to initiate the discharge. The distributor gap generally sparks across before the spark plug, at a much lower voltage, and is thus generally the weaker of the two sources.
Suppression or radio frequency interference is highly desirable and is particularly so with the more recently developed capacitor discharge ignition systems which are generally accompanied by high levels of such interference. Recommendations as to the limits on the levels of such interference have been established by the Society of Automotive Engineers (SAE); and by the Comite International Special des Perturbations Radioelectriques (C.I.S.P.R.). Most of the nations of the world have adopted one or ther other of these recommendations in establishing official limits on the levels of radio frequency interference that shall be allowed.
Automobiles and like vehicles have typically used spark-ignited internal combustion engines operating on the four-stroke cycle, wherein the lubricating oil is kept largely separated from the combustible mixture. Consequently, the spark plugs are normally free of low resistance fouling, which allows such engines to make satisfactory use of inductive ignition systems, which are characterized by a relatively slow build-up of voltage across the spark gap followed by a long duration spark discharge at relatively low currents. Such spark gap and ignition system characteristics have, in turn, allowed the general use of high resistance ignition leads for suppression of the interferences generated by the firings of the distributor gaps and the spark plug gaps.
Outboard motors, snowmobiles, and the like, in the interest of high power output from engines of light weight have typically operated on the two-stroke cycle, wherein the lubricating oil is generally pre-mixed with the fuel and is, therefore, distributed throughout the combustible mixture. Spark plugs in such engines are subject to resistance fouling conditions such that satisfactory operation may require the use of a capacitor discharge ignition system. Such ignition systems are characterized by a very fast build-up of voltage on the ignition coil output, which can rise to a sparking voltage level even if loaded down with a resistance-fouled spark plug such as would render an inductive ignition system inoperative. After the initiation of the spark, a relatively short duration spark discharge at relatively high current follows. Such spark gap and ignition system characteristics have prohibited the use of high resistances in the path between the ignition coil and the spark plug gap, whether in the form of high resistance leads, or high resistance resistor spark plugs, or both.
In general, suppression methods which have proven effective on automotive inductive ignition systems have seriously degraded engine performance when applied to capacitor discharge ignition systems on two-stroke cycle engines, such as outboard motors and the like.
One method which has been suggested to reduce radio frequency interference on outboard motors is an arrangement wherein various lumped-constant suppression elements are incorporated into the ends of the various ignition leads.
At the spark plug end, the lead is fitted with a grounded electrostatic shield in combination with a lumped inductance coil built into the outer end portion of the shield and adjacent to the spark plug. The shield is made of two halves permanently crimped together. A flexible coiled spring about 1.5 inches long enclosing the ignition lead is located on the other side of the inductance coil. A grounding wire extending from the coiled spring is trapped under the shield at the time the two halves are crimped together. By virtue of this construction, a lumped-constant LC filter is formed by the inductance of the inductance coil and the capacitance between the ignition cable conductor and the grounded flexible coiled spring.
At the distributor end, the various ignition leads are fitted with lumped constant resistors. The resistor values are relatively low.
The lumped-constant elements contribute to interference suppression and hold resistance losses to a moderately low level. However, the use of the lumped-constant elements to be incorporated into the wiring system is relatively expensive and is vulnerable to normal difficulties arising from vibration and heat plus the physical stresses accompanying routine spark plug replacement and the like.
Thus, there is a need for a simpler, more sturdy suppression means which does not require lumped-constant elements incorporated in the ignition leads.